I strongly believe that desktop Linux is just waiting for a paid, commercially supported variant that comes with limited, supported hardware options.
That is, like Apple. Vertical integration.
That is, like Apple. Vertical integration.
Unfortunately the Linux kernel itself breaks binary compatibility quite often, so you can't distribute binary blobs if you want to keep up-to-date (and you want that, since newer kernel versions have more hardware drivers). Linux distributions in general have a bad history on maintaining backwards-compatibility.
And without third-party proprietary applications being distributed for this "commercially supported" OS, it's not going to make any inroads.
Actually, if you really care, you can still run a.out and glibc5 ELF userland binaries on Linux. On the other side, any m68k or ppc for classic apps are no-go on OSX, as well as some ppc-for-osx. Also for third party drivers, you need version for specific OSX point release, they are not generally very compatible.
There are also entire classes of hardware that are unusable on OSX, period. (I remember wasting particularly lot of time on trying to get IrDA run on OSX with a little bit more than just IRCOMM. On Linux, it worked out of the box.)